1. Field of the Invention
The present invention relates to fan jet engines of the type having a high by-pass ratio provided by one or more sets of fan blades extending outside the core engine housing and powered by one or more turbines receiving combustion gas generated by the core engine.
2. Description of the Prior Art
High by-pass engines, i.e., engines having a by-pass ratio greater than about 6, become somewhat cumbersome in two ways. With a direct drive from the power turbine to the fan, the turbine diameter and number of turbine stages both increase, and the resulting weight and cost are not attractive. Also, the low pressure compressor or "booster" positioned between the fan blades and the high-pressure compressor requires more axial stages and icing tendencies become excessive.
A typical solution to these problems is to provide a main reduction gear between the fan and the booster so that the power turbine and the booster can run at a high rpm speed while the fan runs at a low rpm speed. This provides a partial solution to the problem in that the power turbine spins faster but still not as fast as desired due to the large annular area on the output stage resulting in high stresses in the last stage rotor.
Counterrotating turbines previously have been proposed for direct drive (i.e. without reduction gears) of counterrotating fans in high by-pass engines as a means of matching fan and turbine speed requirements. Exploitation of this principle has been limited, however, by the torque capacity of the low speed shaft passing through the center of the core engine.